Flow-rate responsive switching-valve for hydraulic circuits



Oct-14,1958 J. L. GRATZMULLER 2,855,945

FLOW-RATE! RESPONSIVE SWITCHING-VALVE FOR HYDRAULIC CIRCUITS Filed Feb. 3. 1954 5 Sheets-Sheet 1 Oct. 14, 1958 J. GRATZMULLER 2,855,945

FLOW-RATE RESPONSIVE SWITCHING-VALVE FOR HYDRAULIC CIRCUITS Filed Feb. 5, 1954 S-Sheets-Sheet 2 Fig.2 7 6 f l 1 21. Z) 12-/ 2s 25" 43 Invent-or Oct. 14, 1958 J. L. GRATZMULLER 7 2,855,945

FLOW-RATE RESPONSIVE SWITCHING-VALVE FOR HYDRAULIC CIRCUITS Filed Feb. 3. 1954 5 Sheets-Sheet 3 In Ve ntor Uct. I4, 1958 J. L. GRATZMULLER 2,855,945 7 FLOW-RATE RESPONSIVE SWITCHING-VALVE FOR HYDRAULIC CIRCUITS Filed Feb. 3, 1954 5 Sheets-Sheet 4 liiiillllL "IIIIIIIIIIJ All Invent w w w by M Oct. 14, 1958 J. L. GRATZMULLER 2,855,945

FLOW-RATE RESPONSIVE SWITCHING-VALVE FOR HYDRAULIC CIRCUITS Filed Feb. 3, 1954 5 Sheets-Sheet 5 Inventor .MMW y rfl x Airorneys FLOW-RATE RESPUNSWE SWITCHING-VALVE FOR HYDRAULIC cmcurrs Jean Louis Gratzmuller, Paris, France Application February 3, 1954, Serial No. 468,010

Claims priority, application France February 7, 1953 Claims. (Cl. 137-108) This invention relates to a valve to be interposed between a source of liquid under pressure and two hydraulic .circuits and having for its purpose, to automatically ensure selective feeding of said circuits with liquid from said source.

More precisely, the purpose of the valve according to the invention is to automatically switch a source of liquid under pressure from one to the other of two hydraulic circuits above a predetermined value of the rate of flow of the liquid through the valve and to automatically switch back the source to the first circuit below said value of said rate of flow.

The invention will be described hereunder in its particular application to hydraulic pumps, and still more particularly in an application in which the automatic switching valve according to the invention is interposed in a hydraulic circuit fed from such a pump, downstream the same, to automatically switch-on said pump in said circuit above a predetermined value of the rate of dischar e of said pump, while below said predetermined value of said rate of discharge, said automatic valve switches the pump to a no-load circuit, e. g. discharging into the feed-tank of the pump.

in this particular application, the automatic switching valve according to the invention ensures starting of the pump in tic-load condition, up to a predetermined working rate, which offers, in practice, several advantages. In particular, it is well-known that the least expensive type of electric motors normally used for driving hydraulic pumps is the asynchronous motor which otters but a very low starting torque. Moreover, the onecylinder pumps, which are also the least expensive, require, due to the irregularity of their resisting couple, a

starting torque which may be three times higher than the average working torque subsequentiy required during normal operation of the pump. One is thus obliged, in ertain applications, to keep the pump permanently working, while an economical use would imply stopping of the pump, once the utilisation pressure is built-up.

The automatic switching valve according to the invention permits stopping a pump whenever desired since its re-starting is made considerably easier, due to its unloading when at rest and as long as it has not reached a predetermined rate of discharge.

An object of the invention is to provide an automatic switching valve comprising a working chamber, an inlet communicating with a source of liquid under pressure, two outlets, a partition movable in said chamber under variations of the rate of flow of the liquid through the valve, and means controlled by said movable partition to selectively establish, below a predetermined value of said flow rate, communication between said source and at least the first one of said outlets and, above a predetermined value of said flow rate, exclusive communication between said source and the second one of said outlets.

Another object of the invention is to ensure the control of said movable partition against the action of elastic means by providing a calibrated passage between the spaces of said working chamber located on either side of said partition.

With this arrangement, the loss of pressure through said calibrated passage creates between both sides of the movable partition a pressure differential which is a function of the rate of flow of the liquid through said calibrated passage. When said rate of flow reaches a predetermined value, said pressure differential becomes capable of overcoming the action of said elastic means and thus displaces said movable partition so as to establish the above mentioned exclusive communication between the source and the second outlet.

A more particular object of the invention, adapted to the special application disclosed, in which said valve is interposed in the utilisation circuit of a hydraulic pump downstream the same, is to provide an automatic switching valve of the type described, wherein said utilisation circuit is that which is switched on by the movable partition when the rate of flow through the valve reaches a predetermined value and wherein the other circuit is a no-load circuit through said pump and its feed-tank.

Another object of the invention is to provide an automatic switching valve of the type described, in which said working chamber and said movable partition are constituted,respectively, by a cylinder and a piston slidably mounted in said cylinder.

A further object of the invention is to provide such an automatic switching valve in which the above mentioned calibrated passage as well as a passage provided for allowing normal flow of the liquid through the valve above the predetermined switching value of the rate of flow are provided through the piston proper.

Still another object of the invention is to provide an automatic switching valve of the type described in which said movable partition opens the communication between the source (e. g. the pump) and the second hydraulic circuit (e. g. the utilisation circuit of the pump) only when it has been displaced by a certain distance further than the position in which it interrupts the communication between said source and the first hydraulic circuit (e. g. no-load circuit of the pump).

Still another object of the invention is to provide the abovementioned calibrated passage in the shape of a bore provided with a series of successive partitions, each provided with a calibrated hole, the holes of two successive partitions being preferably shifted transversally with respect to one another.

Another object of the invention is to provide a hydraulic unit comprising a hydraulic pump driven by a motor, at least one hydraulic pneumatic power accumulator fed from said pump, an automatic switching valve interposed in the feed circuit of said accumulator downstream said pump, a non-return valve between said automatic valve and said accumulator to prevent liquid from flowing back from said accumulator, a no-load circuit for said pump through said automatic valve, and automatic means to stopsaid pump as soon as a predetermined pressure is built-up in said power accumulator and to start said pump motor as soon as said accumulator pressure falls under a second predetermined value lower than the first one.

A more particular object of the invention is to useas automatic means for the purpose described in the last paragraph, a pressure responsive controlling member adapted to actuate two contacts controlling, through suitable relay means, the pump motor, one of said contacts exclusively controlling the starting of the pump motor while the other one exclusively controls stopping of said motor.

Still another object of the invention is to provide a hydraulic unit comprising a hydraulic pump driven by a motor, at least one hydraulic jack fed from said pump and acting against a return-force, an automatic switching valve, as described above, interposed in the feeding circuit of said jack, a no-load circuit for said pump through said valve, automatic triggering means for periodically starting said pump motor and automatic interrupting means to stop said motor after a predetermined stroke of the jack.

This arrangement permits obtaining with any desired frequency the following cycle of operations: Starting of the hydraulic pump without objectionable starting torque, on-switching of the automatic valve, active stroke of the jack, stopping of the pump, unloading of the pump, automatic resetting of the jack into its original position.

This particular mode of operation is permitted by the fact that, in this application, the utilisation circuit is not provided with any non-return valve and by the fact that the jack feeding circuit which is fed from the pump when the no-load circuit is interrupted, finds itself in communication with the feed-tank of the pump when said no-load circuit is established.

A suitable choice of the parameters of the automatic switching valve then permits predetermining the timing of each operation of the above detailed cycle.

Other objects and advantages of the invention will be apparent from the following detailed description together with the accompanying drawings submitted for purposes of illustration only, and not intended to define the scope of the invention, reference being had, for this purpose, to the subjoined claims.

In these drawings:

Figs. 1, 2 and 3 are axial sectional views of an automatic switching valve according to the invention shown in three different positions corresponding to no-load condition, closing of the unloading outlet, and normal working, respectively.

Fig. 4 is an axial sectional view of a modification of the valve shown in Figures 1-3 in the form of a simplified embodiment.

Fig. 5 is a diagram of a hydraulic unit for controlling the pressure in a hydraulic power accumulator, said unit including an automatic switching valve according to the invention.

Fig. 6 is a diagram of a hydraulic unit for periodically actuating a hydraulic jack under the control of an automatic switching valve according to the invention.

Fig. 7 is a wiring diagram of an electric circuit adapted to be used for automatic control of the hydraulic unit shown in Fig. 5.

Fig. 8 is a wiring diagram of an electric circuit adapted to be used for periodic control of the hydraulic unit shown in Fig. 6.

In the example shown in Figs. 1 to 3, the automatic switching valve according to the invention comprises a cylinder 1 tightly closed at both ends in which is slidably mounted a piston 2 separating cylinder 1 in two compartments 3 and 4, the volumes of which simultaneously and inversely vary as a function of the position of piston 2. Compartment 3 is connected, through an inlet 5, with a source of liquid under pressure, which, in this description, will be assumed to be a hydraulic pump. Compartment 4 communicates, on the one hand, with the utilisation circuit of the pump through an outlet 6 and, on the other hand, through another outlet 7, e. g. with the feed-tank of said pump. A calibrated passage 8 establishes a permanent communictaion through the body of piston 2 between compartments 3 and 4-, which may be also in communication, under the control of a valve movable member such as a ball 9, through a second passage 111012 including an axial port provided with a shoulder 13 constituting a seat for ball 9 and an axial recess 12 coaxial with port It) and communicating therewith. Said recess constitutes a housing for a gudgeon-pin 21 which is fitted throughand protruding on either side from a rod 14 carrying at its outer end a suitable valve movable member 15 adapted to control out: let 7.

In the example shown, a portion of increased diameter 16 of rod 14 is slidably guided in a bore of an axial boss 17 provided in the bottom of cylinder 1 and communicating through a port 18 with the outlet 7 proper. A strong spring 19 continuously urges piston 2 towards inlet 5. In the example shown, spring 19 is interposed, for this purpose, between that head of cylinder 1 through which are provided the outlets 6 and 7 and a stirrup member 26, of which the displacement towards inlet 5 is limited by an abutment shoulder 28 of cylinder 1. The stirrup member 20 is provided with an axial hole 22 through which rod 14 is accommodated and the diameter of which is smaller than the length of gudgeonpin 21.

The movable member 1416-l5 is urged outwardly from the piston by means of a spring 23 so that the active end 15 of said movable member tends to be held in seated position under the action of said spring 23 when the stirrup member 20 does not positively lift said movable member active end off its seat.

Rod 14 is provided at its inner end located inside piston 2 with an axial extension 24 constituting a pushing finger, the function of which is to lift ball 9 off its seat 13 a certain time after outlet 7 has been closed, to cause opening of passage 111012 and, hence, to establish a communication through piston 2 between inlet 5 and the utilisation outlet 6.

In the example shown, the calibrated passage 8 which ensures a displacement of piston 2 and, hence, the operation of the valve, as a function of the rate of discharge of the pump, is constituted by a blind axial bore, the outer end of which opens in that end of piston 2 which faces inlet 5 and the inner end of which communicates through ports 25 with compartment 4. In this example, passage 8 is provided with a succession of cross partitions 26 spaced axially and each provided with a small calibrated hole 27. In the constructive embodiment more clearly shown in Fig. 3, the calibrated holes 27 are staggered in such a manner that the jet of liquid from a given hole cannot penetrate directly into next following hole.

This device operates as follows:

At rest (position shown in Fig. l), the stirrup member 20 is held against its abutment 28 by spring 19. Rod 14 of the movable member controlling the no-load outlet 7 is held in abutment against stirrup member 20 by spring 23. As the hydraulic pump is started, it feeds chamber 3 first at a low rate. The liquid flows through calibrated passage 8 into compartment 4. The rate of discharge of the pump increases. When said rate reaches a predetermined value, the loss of pressure through the callbrated passage 8 causes a displacement of piston 2 against the action of spring 19 and hence the closure of outlet 7 by the active end 15 of the movable member 1446-15 (position shown in Fig. 2). From this very moment, the liquid contained in compartment 4 can no longer escape through outlet 7 and must pass only through outlet 6. Piston 2 is further displaced in the same direction and ball 9 is brought into contact with the pushing finger 24 and then lifted 01f its seat (position shown in Fig. 3).

Now, the liquid can flow through the large passage 111012 into compartment 4 from which it is forced under the action of the pump towards the utilisation circuit through outlet 6. This is true as long as the rate of discharge of the pump does not fall under a predetermined value. When said rate of discharge becomes lower than said value, piston 2 is returned towards inlet 5 under the action of spring 19 which has become preponderant over the pressure differential between the two sides of piston 2. During said displacement if piston 2, the movable member 15 is first held on its seat by spring 23, while ball 9 is being seated under the action of the liquid flowassesses ing through passage 11 due to the withdrawal of piston 2 with respect to finger 24 (position of Fig. 2).

Piston 2 is further withdriwn. The gudgeon-pin 21 comes into abutment with the stirrup member 20, whereupon the latter lifts the end of the movable member 14-16-15 off its seat. Compartment 4 is thus brought into communication with outlet 7 and, hence, with the noload circuit of the pump. The spacing. between the two positions of piston 2 which ensure the closing of ball 9 and the opening of the movable member 14- ll6l5, respectively, avoids any unwanted opening of the latter when the rate of discharge of the pump is irregular, which is the case in a one-cylinder pump.

A further advantage of an automatic switching valve according to the invention, when connected to the outlet of a hydraulic pump, is that of ensuring automatic repriming when the pump has been unprimed due to the presence of air in its working chamber.

It will be understood that if back pressure of a certain value exists downstream the pump, it often happens that the pump is not capable of opening its delivery valve by compressing the mass of air in its working chamber if the volume of the same is substantial. This condition of unpriming has for its effect to stop the output of the pump. Now, the automatic switching valve according to the invention is precisely based for its functioning on the rate of discharge from the pump. Obviously, the fact in itself of having this discharge stopped by said unpriming of the pump establishes automatically the communication of the outlet of the pump with a zone of low pressure, to wit the feed-tank. This, in turn, eliminates the obstacle to the opening of the delivery valve of the pump by suppressing the counter-acting back-pressure. The delivery valve opens, the air escapes towards the low pressure zone, the liquid is sucked again in the working chamber of the pump, the flow rate is re-established and the automatic valve acts again in the opposite direction to reestablish the communication between the pump and the utilisation circuit. It is obvious that no pressure-responsive valve is capable of ensuring such an automatic repriming.

In the alternative embodiment shown in Fig. 4 which is adapted to high-pressure pumps, the calibrated passage has been considerably simplified: It is constituted by a single throttling portion 8a. On the other hand, the passage which is provided to ensure normal rate of flow towards the utilisation circuit is constituted by an annular space lila comprised between the wall of an axial bore provided in piston 2a and the outer wall of a tube 29 in which the rod 14a of the unloading movable member 15a is slidably mounted.

In this simplified embodiment, the ball 9 has been sub stituted for by an annular shoulder 39 of the above-mentioned tube which abuts, in its closing position, on the edge of bore llla.

This alternative valve operates broadly in the same manner as the embodiment described with reference to Figs. 1, 2 and 3.

In the diagram of Fig. 5 is shown a hydraulic unit which is adapted to maintain above a predetermined value the pressure in a hydraulic-penumatic power accumulator 31. For this purpose, as soon as said pressure falls below said value, suitable means, an example of which will be described in detail hereunder, ensure automatic starting of a motor 36 driving a hydraulic pump 32 that feeds accumulator 31 through an automatic switching valve 33, such as described above, a non-return valve being provided to prevent the liquid in the accumulator 31 from flowing bacit towards the feed tank 34 of pump 32. The starting of pump 32 under the action of motor 36 offers no difliculty due to the fact that as long as the rate of discharge of pump 32 does not reach a predetermined value, the automatic switching valve 33 connects the outlet of said pump with its feed tank, so that said pump runs in no-load condition. A certain time after the nor- 6 mal working rate of discharge of the pump has been reached, the automatic valve 33 first interrupts {116300111- munication of the pump with the feed tank 34 and then establishes a communication between pump 32 and the accumulator 31 through the non-return valve 35, so that said pressure in said accumulator progressively builds up. When said pressure reaches a second predetermined value higher than the first one, other automatic means, an example of which is also described in detail hereunder, stop motor 36 and, hence, pump 32, so that the automatic valve 33 switches pump 32 again onto its no-load circuit. The unit remains at rest as long as the pressure in accumulator 31 remains above the minimum value at which it is to be maintained. If the pressure falls undersaid value, pump 32 is started again and so on. There are shown at 75 and 76 ducts 4 connecting the accumulator 3i. and the feed tank 34, respectively, with the utilisation circuit of accumulator 31.

An embodiment of automatic means for starting and stopping an electric motor, which may be used in the above-described hydraulic unit, is shown in Fig. 7.

Accumulator 31 is provided with a pressure measuring device which may be for example a pressure indicator of the type described in U. S. Patent No. 2,597,724 granted to applicant.

The movable member of such a pressure indicator is shown at 37. Said movable member is displaced upwardly (as shown in the drawing) when the pressure in accumulator 31 increases.

In the example shown in Fig. 7, the electric motor 36 is fed with three-phased current from contactors 39--4fidl through contacts 42-43-44, respectively. These contacts are completed by a fourth one, 45, the function of which is to close a holding circuit. The four contacts 42 to 4.5 are controlled simultaneously by an electro-magnet 46, the operating circuit of which passes through two normally closed contacts 474 connected in series, said contact being held in this normal closed position under the action of spring leaves 49-5tl, respectively. These contacts 47 and 48 are of the snap 'type and they are preferably constituted by circuit breaking micro-switches. A hand interrupting switch 51 is also incorporated in the operating circuit of electro-magnet 46. A holding circuit passing through contact 45 shunts contact 47 so that when contact 45 is closed, the operating circuit of electro-magnet 46 remains completed whatever the positionof contact 47. The opening of the latter is controlled by a pushing rod 52 carried by a plate 53 fast with the pressure responsive movable memher 37 while a second pushing-rod 54 similarly controls the opening of contact 48 of the other micro-switch. At rest, the difference of level between the active end of pushing-rod 52 and the actuating member of contact 47 is smaller than the difference of level between the active end of pushing-rod 5'4 and the actuating member of contact 48, so that when the movable member 37 is displaced upwardly (as shown in the drawing), the contacts 47 and 48 are opened successively and in this order, and vice versa. Preferably, the opening rods 52 and 54 are slidably mounted on supports 73-74 the height of which is adjustable and said rods are subiected to the action of small springs 5556, respectively, which permits ensuring a mechanical protection of the actuating members or" contacts 47 and 48. The device is so adjusted that F contact 47 will be opened when the level of the movable member 3'7 corresponds to the minimum pressure at which the accumulator 31 is to be maintained and that contact 48 will be opened for the maximum pressure at which the pump is to be stopped. It is thus possible to vary at will the alternate rythm of operation and resetting periods of the pump without using adjustable mechanisms which are always of a very delicate nature.

This device operates as follows: in the position shown in Fig. 7, the pressure in accumulator 31 is below the lower one of the two critical values. If the hand interrupting switch 51 is closed, since contacts 4-7 and 48 are both closed, the operating circuit of electro-magnet 46 is then completed. Said electro-magnet 46 simultaneously closes the four contacts 424344 :5. The three first of said contacts ensure completion of the threephase feeding circuits of the windings of motor 36 and the fourth one completes the holding circuit of the electro-magnet 46. Motor 36 is started and drives pump 32 that feeds accumulator 31, as described to reference with Fig. 5. The pressure in accumulator 31 increases. When said pressure reaches the lower one of the two critical values, the pushing-rod 52 opens the contact 47. The electro-magnet 46, however, is still operated through its holding circuit 45. When the pressure reaches the upper one of the critical values, contact 48 is opened in turn under the action of the pushing-rod 54 and electromagnet 46 releases. Contacts 42 to 45 all open simultaneously, whereby the windings of motor 36 receive no more current, so that said motor stops.

After a period of longer or shorter duration, if the pressure in accumulator 31 decreases, it falls first below the upper critical value and contact 48 is brought into its normal closed position under the action of spring-leaf 50, while contact 47 is still held opened by pushing-rod 52, so that nothing occurs as yet. However, if the pressure falls below the lower critical value, the pushing-rod 52 releases contact 47 which is then in turn brought back into its normal closed position by its spring-leaf 49, so that the operating circuit of electro-magnet 46 is completed again, as described above. Motor 36 and, hence, pump 32 are restarted and a new cycle of operation is initiated.

In Fig. 6, is shown a hydraulic unit for controlling a jack 57. As the unit is set into operation, jack 57 effects a well-defined active stroke during which it displaces a load 58. Then the jack is automatically reset into its original position under the return force of said load. This hydraulic unit comprises a motor 59, e. g. an electric motor driving a pump 32a, the starting of the motor being triggered, e. g. by means of a timing contactor which permits etfecting successive displacements of the load with any desired frequency. As in the previously described unit, an automatic switching valve 33a is interposed between pump 32a and jack 57. The starting of pump 32a under the action of motor 59 is made very easy by the presence of said automatic valve which only switches the pump on when its rate of discharge has taken its normal working value. Once jack 57 has effected its predetermined active stroke, automatic means, an example of which will be described in detail hereunder, stop motor 59 and hence pump 32a, the automatic valve 33a automatically sets jack into communication with the feed tank 340 of pump 32a, so that load 58 becomes capable of resetting jack 57 into its original position. The communication of jack 57 with tank 3 m is ensured by the fact that the utilisation circuit does not comprise any nonreturn valve.

The automatic means for starting and stopping motor 59 may be designed, e. g. in the shape shown in Fig. 8. In this example, the energizing circuit of motor 59 passes through a rotating contact 69 driven e. g. from a clock and cooperating with two pairs of diametrically opposed stationary contacts of material angular size or segments. The stationary contacts 61-63 of one pair are connected with one of the fixed contacts 65 of a two-way ratchetswitch of which the fixed contact 66 diametrically opposed to 65 is connected to the current source provided for energizing motor 59. Similarly, the stationary contacts 62, 64 of the clock contactor are connected with a third fixed contact 67 of the ratchet-s itch of which the fixed contact 63, which is diametrically opposed to 67, is also connected to said source. The rotary contact of the ratchet-switch comprises a first conducting arm 69 and a second insulating arm 70. Said arms are mounted in cross-like fashion and they are keyed on the same shaft as the ratchet-wheel 78 which is controlled by a catch 71 in the usual manner. The catch 71 is carried by the rod of jack 57, the whole assembly being so designed that, whenever the ratchet-switch is actuated by the jack rod, the rotary member 69-70 of said switch is rotated by under the control of elastic locating means 77.

This device operates as follows:

In the position shown in Fig. 8, the rotating contact 60 of the clock-switch is on the stationary contact 61 and an energizing circuit is completed for motor 59 through the contacts 63' and 61 of the clock switch and through the conducting arm 69 of the ratchet-switch, provided a hand-interrupting switch incorporated in said energizing circuit, is closed. The jack 57 is fed from the pump 32a driven by motor 59, so that said jack displaces load 53. It is to understood that the clock switch must be designed in such a manner that arm 60 does not leave the contact segment 61 before the jack 57 has completed its active stroke. At the end of said stroke, catch 71 actuates the ratchet-switch and the connecting arm 69 is rotated by 90, thus interrupting the energizing circuit of motor 59 while connecting the next following contact segment 62 with the current source. Motor 59 and, hence, pump 32a are stopped. The automatic switching-valve 33a sets jack 57 into communication with the feed tank 340 and the load 58 brings jack 57 into its original position again. The unit then remains at rest until the rotating contact 60 of the clock-switch reaches the stationary contact segment 62 which, as stated above, has been and is connected with the current source. A new energizing circuit is then completed for motor 59 through the contacts 60 and 62 of the clock switch and through the connecting arm 69 of the ratchet-switch. Pump 32a is started anew, the jack efiects another active stroke and so on.

It is to be understood that the invention is not limited to the examples described and shown and that it may be effected with numerous modifications within the reach of those skilled in the art without departing from the spirit of the invention.

In particular, the movable partition which is displaced as a function of the rate of flow through the valve to control the automatic switching may be designed in any desired manner; the two passages may be arranged outside the cylinder, the larger passage may be provided with closing valve means calibrated at a predetermined pressure; it may be also controlled directly by the movable partition within the scope of the invention.

What I claim is:

1. An unloader valve comprising, a body having a chamber therein, a movable partition in said chamber to divide the same into first and second compartments, said first compartment having an inlet adapted to be connected to a pump outlet and said second compartment having a delivery port and a valve controlled return port, spring means to urge said movable partition towards said inlet, a continuously open calibrated passage from said first to said second compartments and means operatively connected with said movable partition to close said valve of the return port and to establish a further communication between said first and second compartments upon displacement of said partition away from said inlet to a predetermined position by an increase in the rate of fiow into said first compartment.

2. An rnloader valve according to claim 1, in which said chamber is constituted by the inner space of a cylinder and in which said movable partition is constituted by a piston slidably mounted in said cylinder.

3. An unloader valve according to claim 2, in which said piston is provided both with the continuously open calibrated passage and with another larger passage controlled by said means operatively connected with said movable partition which passage constitutes said further communication.

4. An unloader valve according to claim 3, in which said means are constituted by a first movable member,

slidably mounted on said piston, adapted to cooperate. with a seat to control said return port and provided with elastic means to urge said movable member outwardly from said piston, and a second movable member cooperating with a seat to control said larger passage, said first movable member being adapted to cause opening of said second movable member when said piston is displaced in said cylinder by a given distance further than the position in which said first movable member is brought into contact with its seat.

5. An unloader valve according to claim 1, in which said calibrated passage is constituted by a bore provided with a succession of cross partitions, each drilled with a calibrated hole, the calibrated hole of a given partition 10 being shifted transversally to said bore with respect to the calibrated hole of the immediately adjacent partition.

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